Soldering machines of this type include a middle soldering tunnel enclosing a soldering chamber between an entry tunnel and an exit tunnel. A conveyor arrangement transports the work pieces to be provided with soldered connection through the entry tunnel, through the soldering tunnel, and through the exit tunnel, whereby the conveying path inclines upwardly in the conveying direction within the soldering chamber. A conduit for a protective gas opens into the soldering chamber. The gas is preferably nitrogen. A tub for a molten pool of solder material is arranged below the conveying path in the area of the soldering chamber. The tub is provided with a heating arrangement and with at least one circulating pump. At least one soldering nozzle with an upwardly directed nozzle opening is arranged within the soldering chamber for producing a swell out of the circulated solder material for contacting the work pieces caused to move above and past the nozzle opening.
The just described soldering apparatus with a protective gas atmosphere in the soldering chamber is known from German Utility Model Publication (DE-GM) No. 8,520,254. Atmospheric oxygen is excluded from the soldering location by means of the protective gas, whereby the soldering quality may be considerably increased because the atmospheric oxygen would lead to an embrittlement of the surfaces of the soldering points or locations. Furthermore, the apparatus has a lengthwise extension due to the sequential arrangement of the tunnels one behind the other thereby achieving a high soldering output capacity because all of the soldered connections to be formed on the downwardly slanting backside of circuit boards, are formed as they pass by the swell of solder material, whereby the circumstance that the solder is only deposited on the metallic conductors passing through the circuit board to its backside, is utilized.
In the known soldering apparatus nitrogen to which, however, hydrogen has been added is used as a protective gas. This hydrogen has a reducing effect and can bond with any oxygen still remaining in the soldering chamber in spite of the protective gas atmosphere. However, at the entry end and at the exit end of the tunnel arrangement the danger exists that an explosive mixture will be formed due to mixing with air. For this reason sluice locks are provided at the ends of the tunnel arrangement and the sluice lock chambers are emptied of air or protective gas by being pumped out after each sluice lock operation. As a result, the apparatus becomes complicated, whereby the output capacity is comparatively small because the need to pass the work pieces through sluice locks does not allow a continuous operation of the conveyor arrangement with a high velocity matched to the soldering operation.
A further disadvantage of the known apparatus exists in that the soldering tunnel comprises a housing extension in its middle region with a comparatively large soldering chamber. A tub for the molten pool of solder is located in the soldering chamber. This housing extension not only contradicts the compact construction, but also leads to a relatively large soldering chamber volume and therewith protective gas volume. Furthermore, a relatively high temperature exists in the soldering chamber because the molten solder material is applied at approximately 250.degree. C. Correspondingly, all the components arranged for the tub of the molten pool and within the soldering chamber such as the drive means for circulating the solder material, and, if necessary, means for the height adjustment of the tub with the soldering nozzle, are subjected to these high temperatures which has disadvantageous effects.